High brightness digital display system

ABSTRACT

A display system using red, green, blue, and white light. The system derives data for the white portion of a color wheel or a white device from the red, green and blue data. The white portion of the color wheel is controlled as if it were another primary color on the wheel. Errors are prevented by a correction applied if the unfiltered light from the source has a different color temperature than the white light produced using the red, green and blue segments of the color wheel, or the devices for those colors. Analysis is performed on the data to determine if white light is necessary to be added to each frame of data.

This application claims priority under 35 U.S.C. §119(c)(1) ofprovisional application Ser. No. 60/048,167, filed on May 30, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to display systems, more particularly to displayssystems that generate images using colored filters.

2. Background of the Invention

One of the most common display devices is the cathode-ray tube (CRT),which generates colors from phosphors. The phosphors luminesce whenstruck by a stream of energy from the cathode-ray tube. The color thatis produced depends upon the frequency of the energy.

A class of newer display systems do not use CRTs, but create imagesusing an x-y grid of individually controllable elements. These devices,spatial light modulators, typically have one or more elements on theirgrid that correspond to a picture element (pixel) of the final image.They typically create color images by changing the color of the lightthat strikes the element, or the light transferred by the element to thedisplay surface. The appropriate combination of colors, typically red,green and blue, and amounts of each color are determined for each pixel.The elements are then controlled to produce the proper amount of eachcolor during a display frame time to allow the eye to integrate that mixof colors into the proper color.

The colored light can be produced in several ways. The system could usedthree separate devices, each with their own light sources of theappropriate colors. Alternately, the system could use three devices, buthave one light source, with beam splitters splitting the appropriatelight color prior to striking the device for that color. This lastembodiment is more common, since having to use three light sources raisethe cost of the system too much to make it practical. A system usingthree devices will be referred to as a spatial coloring system.

Use of one device and light source, or two devices and either one or twolight sources are also possible. These systems can be more desirable,since having fewer devices means less cost. However, in order to producetwo or three colors during one frame time requires some type of timedivision of the frame among the colors. Using two devices means that onedevice will have to produce two colors, and one device systems have toproduce all three colors on that device. The time allocated for anycolor must be shortened. This type of filtering system will be referredto as a temporal coloring system.

Using one light source, whether it be for a spatial or temporal coloringsystem, reduces the available light for each color. For three-devicesystems, each device gets 33% (⅓) the light from the source, as does aone device system. Systems with two devices have one device that can getup to 50% of the light for the device processing one color, and aslittle as 25% of the light for the device handling two colors.

One solution for this problem has been suggested in U.S. Pat. No.5,233,385, titled “White Light Enhanced Color Field SequentialProjection,” and assigned to Texas Instruments, Incorporated. In thatpatent, the system added white light as either a segment of a colorwheel, for temporal coloring systems, or a fourth device, for spatialcoloring systems. However, the white portion of the frame was merely toadd an overall brightness “floor” to the image. Depending upon thecharacteristics of the light source, the filters used, and the imagebeing projected, this result can cause a washing out of the colors,especially in the high brightness areas of the image. Too much whitelight added to any of the primary colors causes that primary to becomealmost a pastel. However, the need to add brightness to the overallpicture remains a problem.

Therefore, a solution is needed that allows addition of white light toincrease overall brightness but that controls the white light to preventwashing out of the colors.

SUMMARY OF THE INVENTION

One aspect of the invention is a color display system which adds whitelight to the imaging as a primary color. The system uses a processingblock for determining a white signal from the red, green and blueinputs, then uses that signal to modulate a spatial light modulator inaccordance with the white light signal. In a color wheel system, thecolor wheel is modified to include a clear segment. The modificationincludes taking into account the positioning of the clear segment tomitigate any possible artifacts that might occur. In a multiple devicesystem, the signal is used to operate a separate modulator for the whitelight. Circuitry is included that compares between the white lightproduced by the red, green and blue components and the white lightproduced by the clear segment of the color wheel. This ensures that nocolor shifts or other artifacts occur.

It is an advantage of the invention in that it boosts the brightness ofan image without washing out already high brightness areas.

It is a further advantage of the system in that it can be implementedwith a minimum of extra hardware and can even lower system costs.

It is a further advantage of the system that it tailors the colorfilters to the individual system, making the color efficiencies higher,thereby producing a better image.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and forfurther advantages thereof, reference is now made to the followingDetailed Description taken in conjunction with the accompanying Drawingsin which:

FIG. 1 is a block diagram of a color wheel display system.

FIG. 2 is one embodiment of a color wheel in accordance with theinvention.

FIG. 3 is a schematic representation of a color filtration circuit usedto generate a white component of the color.

FIG. 4 is a schematic representation of a display engine architecturefor an adaptive white enhanced display system.

FIG. 5 is a schematic representation of a sensor board for electronicprogramming of a color wheel using white light.

FIG. 6 is a flow chart of a color wheel calibration procedure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Spatial light modulators that use either color wheels or multipledevices to produce color reduce the amount of time available for eachcolor. In a typical three-color wheel, each color gets 33% of the frametime for display, resulting in 33% of the total light available during aframe period being used for that color. In multiple device systems,unless there are three sources, each color gets at most 50% of the lightavailable from the source during the frame time.

This low amount of light leads to dark images and can cause noticeableproblems in already dim areas of the images. One technique to correctthis problem is the use of a white area on the color wheel, or aseparate device for white light. Currently, these techniques add a baselevel of brightness with the white light and then modulate the colorsegments as normally done. The base level of brightness is notcontrolled, other than to set and then monitor the base level for anyimage. An example of this technique is discussed in more detail in U.S.Pat. No. 5,233,385, titled “White Light Enhanced Color Field SequentialProjection,” assigned to Texas Instruments, Incorporated, andincorporated by reference herein.

One of the problems with this technique is that it is universallyapplied to all areas of the image. It eliminates or minimizes the darkarea dimness, but can make the bright areas of an image appear in almostpastel colors. One aspect of the present invention is that the clear, orwhite (these terms are interchangeable for the purposes of thisdiscussion), segment of the wheel is controlled independently of theother colors, as if it were one of the primary colors (red, green orblue). This allows control of the brightness for all areas of the image,making dark areas appear correctly, while not washing out the brightareas.

The System

Referring now to FIG. 1, a system level block diagram shows the parts ofa color display system 10, sometimes referred to as an engine, used inaccordance with the invention. The chassis 12 has mounted within it alamp 14, a lens 16, which receives light from the lamp 14, and directsit through the color wheel 18. The color wheel 18 is operated by a motor20, and calibrated by use of a sensor board 22. The wheel 18, motor 20and sensor board 22, are all mounted by a bracket 21. The sensor board22 and motor 20 are given operating instructions and communicate withthe system timing and control electronics 24.

This example, for discussion purposes only, assumes a white light sourceand a color wheel. However, the addition of white light to a system isnot dependent upon the light source and the means for producing color.The system could have three colored light sources, such as lasers, threewhite light sources with filters, one light source with filters, orother possible configurations. The use of the term “light source” ismeant to include all of these possible combinations.

The timing and control of the system is critical, since light throughthe color wheel 18 and the relay optics 26, must impinge the active partof the spatial light modulator (SLM) array 28 in the proper sequence.The image created by the array of individual elements on the array isthen projected or displayed. The example of claim 1 involves projectionoptics 30, although the image could be directly viewed. Regardless ofthe final display surface, the electronics 24 must match the events ofthe color wheel 18 to the operation of the SLM 28.

Any variation in the motor speed or calibration of the filter affectswhat color the light is striking the SLM 28, which is also connected tothe electronics 24. If the timing is not properly adjusted, the data forthe red segment, for example, could be on the SLM 28, when the lightcoming through the color wheel is blue.

A second part of the timing and control is the determination of whatareas of the image need how much brightness added during the clearsegment. First, the brightness or white signal must be determined. Thisdetermination varies according to whether the incoming signal is video,which is typically in a YUV format, or computer graphics or data, whichis typically in a RGB format.

The YUV format separates all of the grayscale of luminance(brightness)information into one channel, referred to as Y. The U and Vsignals contain color information. Typically, video undergoes aconversion of the U and V signals into the appropriate levels of red,green and blue (RGB) before being displayed on a SLM. In video, the Ysignal is already provided, although it is used in determination of thecolor-space conversion to RGB. In computer graphics, the Y signal mustbe derived from the RGB inputs, since most computer-generated data is inRGB format. The calculation for each pixel is W=min(R,G,B).

An example of a circuit which performs this derivation is shown in FIG.2. Each of the blocks on the diagram represent function blocks within aprocessor, or can be individual processing elements, evenfield-programmable gate arrays (FPGA) or application-specific integratedcircuits (ASIC). The function blocks will be referred to by their names,rather than their parts.

The signal W is used for two purposes. It determines if gains is to beapplied for each pixel, and it is used in the gain calculation. This isperformed in the Y_DETECT block of FIG. 2. The gain that is actuallyapplied is limited by the amount of white available within a givenpixel. It can also be limited by a maximum gain signal that could be setby the user. The signal C_(max) is defined to be the maximum of anyvalue of color.

if (W≧C_(max))then $\begin{bmatrix}R^{\prime} \\G^{\prime} \\B^{\prime}\end{bmatrix} = \begin{bmatrix}{R + {g( {W - C_{\max}} )}} \\{G + {g( {W - C_{\max}} )}} \\{B + {g( {W - C_{\max}} )}}\end{bmatrix}$

At this point, the RGB signal dynamic range has been increased up to 2times the original signal. While each pixel that exceeds C_(max) hasbeen brightened, the possibility of clipping exists. This is thesituation in which the clear segment is used.

The equivalent brightness or weight of the Y segment can be determinedfor a given RGBY color wheel design. Some predetermined number ofcontrol bits per pixel, Y_LEVEL, are associated to modulate the clearsegment. The white segment could be any number of bits. Implementationshave been done with 1, 2 and 4 bits, although it could have just as manybits per pixel as the primary colors.

For any pixel, the Y signal is enabled when the R′, G′, or B′ signalsexceed the maximum dynamic range of the system. However, these valuesmust be matched in hue and luminance for the potentially large intensityincrease. This is controlled by the Y_THRESHOLD block. The logic in thatblock determines in which “bin” each pixel belongs. The bin is merely agrouping of intensity levels. How much offset is applied at the OFFSETblock of FIG. 2 is determined by which bin a pixel falls into.

The three blocks described above and shown in detail in FIG. 2 is justone example of a brightness control scheme that can be used inaccordance with this invention. For complete understanding of thenecessary information to understand this example, however, a completelist of all of the signals is given below.

Signal Name Description Associated Blocks WCLK Pixel Clock ALLPBC_YMDSEL Y Mode Select Y_DETECT, OFFSET PCB_RWDFEN RGBY Functionenable Y_DETECT, OFFSET PBC_WTEST RGBY Fault Coverage Test Y_DETECTEnable PBC_YMIN Y Gain Threshold Y_DETECT PBC_YGAIN Y Gain Y_DETECTBDP_RED Red Pixel Data Bus Input Y_DETECT BDP_GRN Green Pixel Data BusInput Y_DETECT BDP_BLU Blue Pixel Data Bus Input Y_DETECT PBC_ROFF0 RedOffset Level 0 Y_THRESHOLD, OFFSET PBC_ROFF1 Red Offset Level 1Y_THRESHOLD, OFFSET PBC_ROFF2 Red Offset Level 2 Y_THRESHOLD, OFFSETPBC_GOFF0 Green Offset Level 0 Y_THRESHOLD, OFFSET PBC_GOFF1 GreenOffset Level 1 Y_THRESHOLD, OFFSET PBC_GOFF2 Green Offset Level 2Y_THRESHOLD, OFFSET PBC_BOFF0 Blue Offset Level 0 Y_THRESHOLD, OFFSETPBC_BOFF1 Blue Offset Level 1 Y_THRESHOLD, OFFSET PBC_BOFF2 Blue OffsetLevel 2 Y_THRESHOLD, OFFSET PBC_YRGBL Y Level 1 Threshold Y_THRESHOLDPBC_YRBGL2 Y Level 2 Threshold Y_THRESHOLD RWD_YMAG White MagnitudeOutput Y_DETECT RWD-Rx Red Pixel Data Bus Delay ALL stage x RWD-Gx GreenPixel Data Bus Delay ALL stage x RWD-Bx Blue Pixel Data Bus Delay ALLstage x RGB_MIN Detected Y Component Y_DETECT, Y_THRESHOLD YLEVEL YContent Level Y_THRESHOLD, OFFSET BDP_LSYNC3 Line Synch DELAYBDP_IVALID3 Input Valid Signal DELAY BDP_OLACT3 Overlay Active DELAYRWD_LSYNC3 Delayed Line Sync N-2 DELAY RWD_LSYNC5 Delayed Line SyncDELAY RWD_IVALID5 Delayed Input Valid Signal DELAY RWD_OLACT5 DelayedOverlay Active DELAY

The DELAY block delays the synchronization signals the appropriatenumber of WCLKS, determined by the number of clocks the data is delayedon the main channel data. This ensures proper timing for the generateddata. The output signals are RWD_YMAG, described above, and RWD_RED,RWD_GRN, RWD_BLU, and RWD_RGBY. The RWD_RED, etc., signals are theadjusted pixel data bus output for the colors for the pixel data busthat feeds the data to the SLM. RWD_RGBY is that data for the white orbrightness output.

The Architecture

This processing block is part of an overall control system, anembodiment of which is shown in FIG. 3. The input is received at a videoprocessor, shown in FIG. 3 as block 32. This may be a separateprocessor, or it may be a function group on a shared processor with theother control functions. Examples of the video processing performed atthis processing include color-space conversion, degamma processing anderror diffusion functions. It could also include such things asprogressive scan conversion and sharpening.

One video processing function performed by this block is degammaprocessing. Nonlinear display devices, such as CRTs, require acorrection of brightness/voltage called the gamma correction. This isnormally included in the video signal. Any linear device must remove or‘degamma’ the incoming signal. The processing for the RGB signals todevelop a Y signal must occur after this processing, for linear devices.

Once the color-space conversion is completed, the RGB signals are sentto the RGBY processing block 34. This block contains the logic discussedwith reference to FIG. 2. Notice that this embodiment assumes the datawas received in YUV format and had to be color converted to RGB. Theinput could also be computer graphics, which is already in RGB format.

After the data has been processed into RGBY data, it is sent to aformatter 38. The formatter 38 performs functions converting typicallyrasterized image data into data for the x-y grid on the SLM 28. Thesefunctions communicate and send data along a system bus 48.

The system bus 48 also sends the various control signals necessary tocoordinate the data processing, formatting and movement of the data intime with the appropriate spoke on the color wheel. Necessary to thiscontrol is the system controller 36, which coordinates all of theoperations. The controller 36 needs as one of its inputs, data from thecolor wheel EPROM 46, that has a function that will be discussed withregard to calibration of the color wheel below.

In addition to the master system controller, a separate timing block 40is used to coordinate between the color wheel's movement, its currentposition and the data flow to the SLM 28. The information with regard tothe color wheel is produced by the color wheel motor controller 44,which can also slow down or speed up the motor 20, as necessary. Themotor controller also receives data from the sensor, 22, which will bediscussed with regard to the programming of the color wheel below. Themotor controller could be operated with a feedback loop, a magneticsensor or a spectral sensor to determine the rate of the motor. Thecontroller then speeds up the motor to match the frame rate.

The motor controller is responsible for detecting the frame rate of theincoming video/graphics signal. Based upon the controller'scalculations, an appropriate PWM sequence is selected. If a new sequenceis selected that has different PWM efficiencies, the system controllerreads the appropriate table from the color wheel nonvolatile memory andupdates the necessary RGBY coefficients.

This example uses PWM sequences as an implementation of the range ofillumination efficiencies of the system. The illumination efficienciescould be implemented in other ways. For example, if the SLM were an LCDpanel, the efficiencies would be based upon the amount of lighttransmitted through a crystal cell and the length of time that cell werein that state. Therefore, while this example uses PWM sequences, thetrue selection is based upon the efficiencies of the various PWMsequences available.

The Color Wheel

The layout of the color wheel itself has an impact on the functioningand interaction of various components of the system in FIG. 3. Oneexample of a color wheel layout is shown in FIG. 4. While this is onlyintended as an example, it is one that works under several constraints.

Some of the constraints under which a color wheel is designed are:brightness; white-point; rotation speed; and flicker performance. Withregard to brightness, the overall system brightness must be increased toa level that makes the trade off of color saturation of the primarycolors worth it. The design shown in FIG. 4 increased the color wheelSLM system efficiency of over 40%.

Current color SLM systems have a white point that has a slight cyan-tintassociated with it. That is, the white of a system using a RGB onlycolor wheel is not pure white, it has an imbalance that makes it have aslight cyan tint. Using a clear segment off a color wheel moves thesystem white point towards a purer white reproduction. The clearsegments have a dichroic filter applied to them in this example suchthat the lamp white point is more closely matched to the white pointcreated by the RGB segments. However, there may not be a need for anyfilter to be applied to the white light segment.

Even with this correction, flicker is noticeable at frame rates of 50Hz, as is used in Europe. The color wheel allows a 4:3 frameup-conversion which creates a refresh rate of approximately 68 Hz. Since480 degrees of the color wheel are presented during a given refreshtime, a constant portion of the display time needs to contain data fromthe clear segment. If a design were not to take this into account, someframes would be brighter than others and have the effect of creatingnoisy areas in portions of the image that utilized the white segment.

The constraints of rotation speed and flicker interact. In currentdesigns, each primary color has 120 degrees. This causes flicker in thegreen segment at display rates of 60 Hz or less. The use of a 40 degreeclear segment 50 that is 180 degrees opposite the green segment 52, plusthe reduction in size of the green segment from 120 degrees to 100degrees, has been proven to reduce the amount of flicker by 30%.

Even with this correction, flicker is still noticeable at frame rates of50 Hz, as is used in Europe. The color wheel allows a 4:3 frameup-conversion. The cause of the flicker is when a frame uses only RGBbecause the threshold was too low to require the addition of Y, but thenext frame uses a white-bit (RGBY data) on the next frame, that pixelwill be momentarily very bright. This is typically observed as a wave ofsparkling pixels.

A comparison of two different modes of operating a color wheel are shownbelow. R, G, and B designate 120 degrees of those colors. The letters r,g, and b represent 60 degrees of those colors. The letter w represents20 degrees of white, and W represents 40 degrees of white.

Frame A Frame B Frame C Current brGrbbr GrbbrG rbbrGrb Error rb G BwRGBY wRwGBwwR wGBwwRwG BwwRwGBw Error Rw Gw Bw

In the current design, the sparkling appears in Frame C where there isadded White, where there was no added white in Frame B, and where thereis 120 degrees of green between Frames A and B. In the RGBY case, eachframe has equal amounts of white energy, so no flickering or sparklewill occur. The extra white energy can be compensated for by reducingthe white gain ¼ or increase the RGB offset values by 4/3. The w of theabove sequences is shown in FIG. 4 as the 20 degree clear segment 54.

Use of the color wheel in FIG. 4 presents a unique set of artifactconcerns. The main artifact occurs along intensity boundaries (wherethere is a large intensity shift) in an image where the white segment isenabled. Another constraint that can lead to artifacts is the use of24-bit resolution, 8 bits for each color. In some systems, dithering isperformed on bit-planes of each image. A bit-plane is a plane of data,each bit of which comes from the digital word for each pixel on theimage. All of the bits in the bit-plane have the same binarysignificance.

The dithering of one or more bit-planes of image data reduces the numberof bits needed to represent an image in memory. It can also reduce thenumber of clear or white bits need for the clear segment.

These implementation factors can have two visual artifacts. A hue and/orluminance shift may occur if the clear-segment color varies or isimproperly calibrated. The artifact appears as a step function orcontour line, especially in a scene such as skies or sunsets. The otherartifact appears due to pulse-width modulation (PWM) of the clearsegment. It manifests as a flashing on the boundaries when the clearsegment is enabled.

These PWM artifacts can be corrected by spatially diffusing them. Theerror diffusion aspect of the processor 32 can eliminate some of theseproblems. Use of a single red segment on the color wheel, as opposed tobreaking the segment into two segments, allow the bit patterns to bemore optimally placed. Linearity also improves since the opticaltransmission variations across a single dichroic element aresignificantly less than those across two elements. This will mitigatesome of the errors that occur, including those due to variation orincorrect values in the clear segment correction.

Programming the Color Wheel

As mentioned above, and reference earlier, it is necessary to ‘program’the color wheel. The Y signal derived from the RGB inputs above may bedifferent than the white produced by the lamp through the clear segment.The system can generate correction coefficients that account for thesedifferences. In order to associate these correction coefficients suchthat any color wheel module can be used in any projector, thecoefficients must be stored on the color wheel itself, or an indicationof the color wheel type must be detectable.

The system shown in FIG. 1 shows one possible implementation of thecolor wheel module. It includes the color wheel, motor, bracket andsensor board. Referring to FIG. 3, it can be seen that an EPROM, or anEEPROM is also part of the module, which communicates with the systemcontroller and other parts of the system. A necessary part of theinformation communicated is the location of the index mark on the colorwheel.

The index mark on the color wheel acts as a timing reference to thesystem electronics, normally the mark occurs just before the red segmentmoves into the filter path. Typically, this mark is sensed by a sensorthat has to be physically moved into the light path to ensure that thetiming pulse occurs at the proper time. The use of a sensor board, anexample of which is shown in FIG. 5, eliminates this need. The sensorcould be magnetic, optical, or even electromechanical to detect theindex mark.

In previous designs, the sensor board was physically moved in order tonullify any mechanical variations in the index mark location relative tothe segment spoke location. Instead of moving the sensor board,nonvolatile memory is programmed with a sequence start delay value. Thisdelay value has the same effect as moving the sensor board, insuringthat the sequence start command for a new PWM sequence occurs at themiddle of the green/red segment boundary. Of course, use of thegreen/red segment boundary is based upon current color wheelconfigurations, it could be adapted to be on any boundary.

As part of the sensor board, an EPROM, EEPROM or other nonvolatilememory is used to store information about the color wheel. Theinformation generated from the processing block of FIG. 2 is stored inthe memory. Also, the correction coefficients, the offset between thelocation of the index mark and the absolute location of the color on thecolor wheel. Other useful information can be stored in the memory,including the serial number, part type or other information that canhelp in the manufacturing process. This nonvolatile memory can be anytime of once-programmable memory, such as a PAL (programmable arraylogic), jumpers, fuses or other switches.

Another possibility of the nonvolatile memory is as an associationtable. A set of the various parameters could be generated for each colorwheel class, then a list of possible classes created. When the class ofcolor wheel is identified, the associated table could then be used,allowing a close matching of the color wheel and the parameters. Thiswill mitigate some of the artifacts discussed above.

A diagram of the sensor board with its nonvolatile memory is shown inFIG. 5. Jumper or other connection 56 allows connection between thesensor board 22, including the nonvolatile memory 46, and the sensor 58,and the main system controller 36. The main system controller 36 isshown in FIG. 3.

While the various signal could be configure in several ways, one exampleof the signals that could be used are shown in FIG. 5. These signals aredescribed below.

Signal Description SCL Serial Clock SDA Serial Data WP Write Protect GNDGround VDD Power Voltage SENSORZ Index Mark Detect A2 Address Select 2A1 Address Select 1 A0 Address Select 0

Color Wheel Calibration

Having discussed at this point the system, the derivation of the Yvalue, the means for timing the color wheel and the place in which tostore correction values, the discussion must turn to how thosecorrection values are determined. A method for calibrating a color wheelis shown as a flow chart in FIG. 6.

In order to properly tune the color wheel system, several parametersmust be set. If they are not, the system will have image artifacts suchas intensity discontinuities, and color shifts. Currently, most systemsdo not use the RGBY system and therefore do not need calibration. Forthe system discussed above, a detailed calibration of the projectorwould be required without some way to approximate the parameters andmaintain the close match between the approximation and the actualcharacteristics of the system.

Five system values are related to the various parameters that must beset for the system. The number of parameters set and their variousimplementations in the calibration procedure is left to the systemdesigner. The five system values to be taken into account during thecalibration process are: Y_(RGB), lumens produced from only the RGBportion of the color wheel; Y_(WS), lumens produced from only the whitesegment portion of the color wheel; CC_(r), the color correction redfactor; CC_(g), the color correction green factor; and CC_(b), the colorcorrection blue factor.

In experimentation it was found that the color correction factors couldbe approximated by a linear relationship to Y_(RATIO), which is theratio of the Y_(WS) parameter to the Y_(RGB) parameter. The perceptualartifacts were evaluated using a measure of “just-noticeabledifferences” or ‘jnd.’ It was found that a jnd factor of 3 or lessprovided acceptable system performance. Using Y_(RATIO) met thiscriterion.

Since all RGBY parameters are related to Y_(RATIO), the system can becalibrated from pre-calculated tables which are indexed by theY_(RATIO). The Y_(RATIO) can be easily measured on most test stationsand the appropriate RGBY parameter set selected. However, this must berepeated for every PWM sequence used in the display engine, since theY_(RATIO) changes due to the SLM electronic efficiencies in red, greenand blue.

This can be further minimized through the use of information on thelight efficiencies for each of the PWM sequences. The Y_(RATIO) can bemeasured at one rate (such as 60 Hz) of the color wheel rotation, andthen scaled to other rates. The measurement of Y_(RATIO)(60) would be asfollows:$Y_{RATIO} = {\frac{Y_{ws}(60)}{{Y_{R}(60)} + {Y_{G}(60)} + {Y_{B}(60)}}.}$

Using the PWM sequences, where PWM_(R), PWM_(G), and PWM_(B) are thelight efficiencies for the red, green, blue, and white segments at thegiven wheel rate, Y′ factors for each color can be computed.${Y_{R}^{\prime} = \frac{Y_{R}(60)}{{PWM}_{R}(60)}};\quad {Y_{G}^{\prime} = \frac{Y_{G}(60)}{{PWM}_{G}(60)}};\quad {{{and}\quad Y_{B}^{\prime}} = {\frac{Y_{B}(60)}{{PWM}_{B}(60)}.}}$

The Y_(RATIO) for other rates can then be computed as follows:${Y_{RATIO}(\theta)} = {\frac{{{PWM}_{WS}(\theta)} \cdot Y_{WS}^{\prime}}{{{{PWM}_{R}(\theta)} \cdot Y_{R}^{\prime}} + {{{PWM}_{G}(\theta)} \cdot Y_{G}^{\prime}} + {{{PWM}_{B}(\theta)} \cdot Y_{B}^{\prime}}}.}$

This process can be even further simplified, if the assumption that thedenominator is equal to PWM_(RGB)·Y_(RGB)′ for the given rate. Thisreduces the measurements necessary to calibrate the system to measuringthe lumens produced by and RGB only white and the lumens produced by thewhite segment.

Y_(RATIO) must be calculated or remeasured for every PWM sequence usedin the display engine, since the Y_(RATIO) since the SLM electronicefficiencies in red, green, and blue. The calculation of Y_(RATIO) forother sequences is made by multiplying Y_(RATIO) by the ratio ofluminous efficiency of the measured sequence by the predicted luminousefficiency of the remaining sequences.

The process of calibration is shown in FIG. 6. During the designprocess, the appropriate Y_(RATIO)s for each color wheel rate areprogrammed into the nonvolatile memory. Once these are programmed, thepossible Y_(RATIO) bins, PWM sequences and system parameters are used atstep 62, in addition to the measurements mentioned above, taken at step64, to select the appropriate color wheel class and its associatedtables of data at step 66. Steps 62, 64 and 66 are typically performedat the calibration station. The final tables are programmed into thenonvolatile memory, in addition to the color wheel rates, and the PWMpatterns. The final result is a display engine with a finely tuned colorwheel system, mitigating any possible artifacts.

An alternative to the use of a calibration station, would be to haveeach system calibrate itself. Calibration could be done in real-time asthe color wheel spins. As mentioned above, one of the important generalparameters is the efficiency of the illumination sequence, as in theexample of the PWM sequence above. One further option would be to use aself-timing cell, where the timing is determined by the value for thatpixel, and the timing is matched to the color wheel rate.

The use of the white or clear segments can add brightness to the overalldisplayed image, but it must be used as if it were another primary colorto avoid washing out the colors. In addition, the control of the whitesegment requires new elements to be added to a display system, includingsensing means. Finally, the wheel must be calibrated to avoid adding anyartifacts to the system. The advantages of brighter images can be hadwithout increases in system cost or size, and can even lower costs andmake the system more efficient.

The above discussion revolves around one implementation of the additionof white light to a display system, those systems which use colorwheels. However, all of the above aspects of the invention relate tosystems which have separate devices for each color, except for thelayout of the color wheel, the sensor interfaces and the calibration ofthe color wheel. However, the calibration procedure could be used toensure that the Y signal derived from the RGB signals is equal to thewhite light produced by the white device.

Thus, although there has been described to this point a particularembodiment for a method and structure for a color display system usingwhite light, it is not intended that such specific references beconsidered as limitations upon the scope of this invention exceptin-so-far as set forth in the following claims.

What is claimed is:
 1. A color display system in which image data isconverted into red, green and blue data prior to display, and displayedon a display system having at least one spatial light modulator,comprising: a) a processing element which derives white image data fromsaid red, green and blue data; b) a source of red, green, blue and whitelight operable to impinge light on the spatial light modulator; c) asystem controller for controlling the timing of the red, green, blue,and white data onto the spatial light modulator, such that theappropriate data for each color is present when that color light isstriking the spatial light modulator; and d) a nonvolatile memory forstoring correction data which corrects for differences between the whiteimage data and the light from the source, allowing the display system toadd brightness to the display system and preventing artifacts.
 2. Thesystem of claim 1, wherein the source of light comprises one white lightsource and a color wheel with red, green, blue and clear segments; andwherein the nonvolatile memory is also for storing a sequence startdelay value that is communicated to the system controller so that thetiming of the data is controlled to coincide with a selected point onthe color wheel.
 3. The system of claim 2, wherein the clear segment isclear glass.
 4. The system of claim 2, wherein the clear segment has adichroic filter applied to tint the white light.
 5. The system of claim1, wherein the source of light comprises a pulsating lamp.
 6. The systemof claim 1, wherein the source of light comprises solid state lightsources producing red, green, blue and white light.
 7. The system ofclaim 6, wherein the light sources are solid state.
 8. The system ofclaim 1, wherein a filter is used for white light.
 9. The system ofclaim 1, wherein no filter is used for white light.